Continuously variable transmission

ABSTRACT

The invention is based on a continuously variable transmission comprising a mechanical power branch and an electronically controlled hydraulic power branch which can be adjusted via a swash plate (7), a summarizing transmission and a multi-stage transmission with hydraulically actuated, force-locking shift elements (11-14). The shift signals of the shift elements (11-14) are generally triggered when a shift or synchronizer speed is reached. Thereafter elapses a response or reaction time and an adjusting time until the shift elements (11-14) are engaged. This results in long clutch shift processes. It is proposed that the response and engagement times of the shift elements (11-14) be compensated by a control unit.

FIELD OF THE INVENTION

The invention concerns a continuously variable transmission having thefeatures stated in the preamble of claim 1.

BACKGROUND OF THE INVENTION

Patent EP 0 429 517 B1 has disclosed a continuously variabletransmission with one hydrostatic and one mechanical power branch. Insuch transmissions, a summarizing transmission adds up the power of bothpower branches and optionally guides it via a mechanical transmission toa vehicle drive. If the mechanical power branch is interrupted by aclutch, the hydraulic power branch transmits the whole power. If thehydrostat is set to zero, the mechanical power branch transmits thewhole power. Between said two operating points, the power distributionis determined by the control of the hydrostat.

The hydrostat comprises one pump and one hydrostatic motor of which atleast one unit is regulatable. Pressure and flow rate or capacity in thetime unit determine the power of the hydrostatic power branch. The ratioof the flow rate of the pump to the capacity of the hydrostatic motoryields the reduction ratio or conversion of the hydrostat.

The higher the power and the greater the conversion range, the largerthe hydrostat must be. Although the hydrostats, especially largehydrostats, are good to regulate continuously, they have a poorereffectiveness and require a higher construction cost in comparison withmechanical transmissions. To keep the hydrostats small, the wholereduction ratio range of the transmission is divided by a multi-stagetransmission into several forward and reverse drive ranges. In eachdrive range, the hydrostat passes through its whole adjustment rangefrom maximum to minimum and vice versa. In the end positions, the shiftelements to be engaged in the transmission attain synchronizer speed sothat it is possible to shift smoothly. To this end as a rule are usedforce-locking, multi-disc clutches which can be shifted by the hydraulicactuation devices.

The hydrostat further reaches or passes through once in each range thezero position in which the whole power is mechanically transmitted withthe best effectiveness. Therefore, transmissions are designed so thatoperating points in which the transmission is very often operated be inspeed ranges of the transmission having a high mechanical transmissionportion of power. By said steps and by numerous drive ranges,transmissions with favorable degrees of effectiveness are obtained. Withthe number of drive ranges and transmission stages, the number of clutchshift processes, of course, increases. Response or reaction andadjusting times of the hydraulic actuation devices result in long shiftprocesses and in losses whereby the effectiveness is deteriorated.

With this background, the problem to be solved by the invention consistsin altogether optimizing, especially abbreviating, the shift processes.

SUMMARY OF THE INVENTION

According to the invention, the problem is solved by the features ofclaim 1, while advantageous embodiments and developments of theinvention can be understood from the subclaims.

Shift signals are usually triggered when a shift or synchronizer speedis attained, thereafter elapse a response or reaction time and anadjusting time until the shift elements are engaged. Therefore, a timeinterval exists between the triggering of the shift signals uponreaching the synchronizer speed and the actual shifting point wherebythe shift process is unnecessarily prolonged and the whole effectivenessdeteriorates.

According to the invention, the time interval between the triggering ofthe shift signals and the actual shift point is compensated by a controlunit. The control unit triggers the shift signal around the time beforethe shift or synchronizer speed is reached so that the actual shiftpoint preferably falls directly on the moment at which the synchronizerspeed is attained. Thereby the shift process is shortened and the totaleffectiveness improved.

According to the invention, the reduction ratio or the moment when theshift signal must be triggered is determined according to the temporarychange of the pivoting angle of a swash plate. It can be calculated withthe following equation: ##EQU1## t_(so) =actual shift point t_(sk)=moment to trigger the shift signal

α₂, α₁ =plate angle at the t₂ or t₁ moment

K_(k) =adaptation factor

In the hydraulic power branch there is likewise a response or reactiontime often also designated as compensating time, between adjustingsignal such as stop, start signal and signal of a swash plate etc. thatchanges the angular speed and the actual reaction of the hydraulic powerbranch. Specially in the case of the stop signal of a swash plateactuation this results in that the latter is adjusted past a targetangle. Thereby results an excess of the hydrostat actuation or anunnecessary pressure peak in a hydrostat working circuit. Thereby energyis unnecessarily lost in the shifting process, structural parts such asseals are unnecessarily stressed. Furthermore, in order to achieve asmooth shift the reaction has to be interrupted by a hydrostat bypassvalve.

It is proposed that the response and adjusting times or compensationtimes of the hydraulic branch be taken into account by a control unit.Specially in the stop signal of the swash plate actuation a signal istriggered around a corresponding time interval prior to reaching thetarget angle so that the swash plate adjusts exactly to the target angleafter the signal. An excess of the hydrostat actuation is prevented anda smooth shifting is possible by means of a hydrostat bypass valvewithout traction interruption.

The moment when the signal must be triggered is calculated, preferablyaccording to the temporary change of the pivot angle, meaning theangular speed, by the following equation: ##EQU2## t.sub.α =moment totrigger a stop signal of a swash plate actuation tα₀ =theoretical momentto trigger a stop signal of a swash plate actuation

α₂, α₁ =plate angle at the t₂ or t₁ moment

kα=adaption factor

In the specification and in the claims are shown and described incombination numerous features. The expert will conveniently regard theindividual features and make added logical combination with them.

BRIEF DESCRIPTION OF THE DRAWING(S)

One embodiment of the invention is shown in the drawing wherein:

FIG. 1 is a diagrammatic representation of a continuously variabletransmission;

FIG. 2 is a curve of an adjusting angle of a swash plate in the courseof time with a moment t_(sk) for triggering a shift signal;

FIG. 3 is a curve of an adjusting signal in the course of time; and

FIG. 4 is a curve of an actual adjusting angle of a swash plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 diagrammatically shows a continuously variable transmissiondriven by a prime mover (not shown) via an input shaft 1 upon which arefirmly mounted two gears 2 and 3 and one other gear 4 is rotatablymounted.

The gear 3 drives, via one other gear 5, an hydraulic variabledisplacement pump 6 with a swash plate 7, the pivot angle a of whichdetermines the flow rate of the variable displacement pump 6. The swashplate 7 is electrically adjusted, its control current T being a measurefor the pivot angle a.

The variable displacement pump 6 belongs to a hydrostat 8 which, besidesthe variable displacement pump 6, has a hydromotor 9 and is a maincomponent part of the hydraulic power branch.

The gear 2 drives a gear 10 at the input of a mechanical power branch(not shown in detail) to which belong a summarizing transmission and ahydraulically shifted mechanical transmission in planetary design.Transmissions with countershaft design can also be used. Gear clutcheswith discs for the transmission are designated with 11, 12, 13 and 14.They connect the transmission in the respective gears with an outputshaft 15 upon which is mounted a drives system bevel gear 16.

The hydrostat 8 is operatively connected, via its output shaft 17, agear 18, the gear 4 and one other gear 19 with the summarizingtransmission.

The swash plate 7 and the gear clutches 11-14 are controlled by one ortwo control units (not shown in detail).

In FIG. 2 is an adjusting angle a of the swash plate 7 plotted in thecourse of time a curve 21. At the t_(sk) moment, a shift signal istriggered to engage one or several shift elements, i.e. the gearclutches with discs 11-14, so that a positive shift point t_(so) isobtained when a synchronizer speed or a shift angle a 20 is reached.

In FIG. 3 is the curve 22 of a control signal plotted in the course oftime. Directly thereunder is shown FIG. 4 for comparison a curve 23 of aswash plate 7 without control unit, according to the invention, and acurve 24 with one such control unit. It is to be noted that only after acertain time 25, the so-called compensating time, does the hydrostat 8react to the control signal. If a stop signal is triggered at the t_(ao)moment, a period of time likewise elapses until the swash plate 7 comesto a stop. If a target angle 26 is reached at the t_(ao) moment, theswash plate 7 adjusts itself past the target angle 26. An excess 27 ofthe hydrostat actuation generates which detracts from the totaleffectiveness. To prevent this, it is proposed that a stop signal betriggered at a moment t_(a) calculated by a control unit so that theswash plate 7 stops directly on the target angle 26, as is shown in thecurve 24.

    ______________________________________                                        Reference numerals                                                            1   input shaft     18    gear                                                2   gear            19    gear                                                3   gear            20    shift angle                                         4   gear            21    curve                                               5   gear            22    curve                                               6   variable displacement pump                                                                    23    curve                                               7   swash plate     24    curve                                               8   hydrostat       25    time                                                9   hydromotor      26    target angle                                        10  gear            27    excess                                              11  gear clutch     t.sub.sk                                                                            moment to trigger a shift signal                    12  gear clutch     t.sub.SO                                                                            shift point                                         13  gear clutch     t.sub.α                                                                       moment to trigger a stop signal                     14  gear clutch     t.sub.αO                                                                      moment to trigger a stop signal                     15  output shaft    T     swash plate control current                         16  drive system bevel gear                                                                       α                                                                             swash plate angle                                   17  output shaft                                                              ______________________________________                                    

What is claimed is:
 1. A process for control of a continuously variable transmission comprising:providing a hydrostatic transmission having a swash plate, providing a summarizing transmission, providing a multi-stage gear clutch with hydraulically actuatable force-locking shift elements (11-14), providing a control unit, adding together a mechanical power branch and an electronically controlled hydraulic power branch adjustable by a swash plate (7) in said summarizing transmission, using said control unit to control said shift elements (11-14) and said hydrostatic transmission, whereinsaid control unit triggering, prior to reaching in said transmission the shift or synchronizer speed needed for a shift point, a shift signal to shift said force-locking shift elements (11-14) so that the actual shift point falls directly on the moment at which the synchronizer speed for a shift is reached.
 2. The process for control of a continuously variable transmission according to claim 1, comprising determining a moment (t_(sk)) for triggering a shift signal of said shift elements (11-14) in accordance with an angular speed of said swash plate (7).
 3. The process for control of a continuously variable transmission according to claim 1, comprising determining a moment (t_(a)) for triggering a stop signal of said swash plate (7) in accordance with an angular speed of said swash plate (7). 